The use of effect pigments, also known as pearlescent pigments or nacreous pigments, in order to impart a pearlescent luster, metallic luster and/or multi-color effect approaching iridescent, is well known. The effect pigments are composed of a plurality of laminar platelets, each of which is coated with one or more reflecting/transmitting layers. Pigments of this type were first based on metal oxides, as described in U.S. Pat. Nos. 3,087,828 and 3,087,829, and a description of their properties can be found in the Pigment Handbook, Volume I, Second Edition, pp. 829–858, John Wiley & Sons, NY 1988. More recently, use of other coating layers to realize optically variable effects have been developed.
The unique appearance of effect pigments is the result of multiple reflections and transmissions of light. The platelet substrate usually has a refractive index which is different from the coating and usually also has a degree of transparency. The coating is in the form of one or more thin films which have been deposited on the surfaces of the platelets.
The addition of the coatings to a platelet so that the luster, color and color homogeneity are maintained is a very complex process and originally, the only platy substrate which achieved any significant use in commerce was mica. Thus, historically, the largest class of effect pigments based on thin film interference were those based on a mica substrate. With the advent of synthetic substrates, e.g. synthetic mica, aluminum oxide, silica, and glass, it became evident that other substrates could be used since each substrate itself contributes certain effect attributes, due to variations in transparency, refractive index, bulk color, thickness, and surface and edge features. Coated substrate effect pigments thus provide different, albeit similar, visual effects when they are identical except for the identity of the material of the substrate because of these considerations.
One of the most important of the effect pigments which is encountered commercially today is titanium dioxide-coated mica, a pigment which is composed of a mica platelet having an adherent coating of crystalline titanium dioxide thereon. The color which is exhibited is a function of the thickness of the coating. The effect pigment has good reflectivity characteristics, high stability to heat and chemical agents and is non-toxic. The titanium dioxide may be in the anatase form as described, for example, in U.S. Pat. Nos. 3,087,827, 3,087,828, 3,418,146 and 3,437,515, or in the rutile crystalline form as described in U.S. Pat. No. 4,038,099.
The metal oxide on the mica substrate has a high refractive index and provides the optical effects, including high luster and reflectivity, coverage, interference reflection color if the metal oxide coating is sufficiently thick, and absorption color if the metal oxide contains a color material. The mica, on the other hand, has a low refractive index and essentially functions solely as a carrier substrate making almost no contribution to the optical effect which is realized. The metal oxide-coated substrate pigments do not present dark absorption colors.
Lustrous interference pigments, presumably dark, have been prepared using chemical vapor deposition techniques as described in the European Patent Nos. 0579091 and 0571836.
A description of the production of dark, in particular black, surface coatings is found in U.S. Pat. No. 5,356,471. This result is achieved by coating a platelet-like substrate with a silane followed by pyrolysis, for example at temperatures which preferably are greater than 700° C. in a non-oxidizing, preferably inert, gas atmosphere. The process leads to the formation of a layer on the surface of the pigment which contains black SiO2 glass containing silicon oxycarbide and/or carbon black. In U.S. Pat. No. 5,286,291, a pigment containing carbon black is achieved by fixing the carbon black on a substrate by means of a surfactant and organic silane compound. The products of these patents are usually characterized by being low in reflectivity and poor in color purity. The use of carbon black is not a very efficient process because the majority of the platelets being treated may not be coated properly. As a result, the above noted methods are characterized by increased production costs, being difficult to precisely control and involving multiple steps.
There is, accordingly, still a need for highly intense interference effect pigments with a dark absorption color which can be made by a simple and cost effective process. It is the object of that invention to provide such pigments and a process by which they may be prepared.